This invention relates to solid state relays and more specifically relates to a solid state relay employing a novel field effect transistor and a novel control circuit integrated with the field effect transistor structure.
Reed relays are well known electromechanical relays in widespread use. Such relays have a limited lifetime, for example, of the order of about one million operations and are relatively large and expensive. Efforts have been made to replace reed relays by relays employing solid state components. These efforts to date, however, have not produced a unit which is generally competitive, in terms of characteristics or economics, with a reed relay type device.
Thus, commercially available solid state relays almost universally use thyristors (SCRs or triacs) as output devices. Anti-parallel connected SCRs are disclosed (for s-c application only) in application Ser. No. 178,689, filed Aug. 18, 1980, in the names of Alexander Lidow and Thomas Herman, entitled "Process for Manufacture of High Power MOSFET with Laterally Distributed High Carrier Density Beneath the Gate Oxide". Thyristors, however, are poor analogs of an ideal electromechanical switch. For example, thyristors have a minimum 0.6 volt on-state voltage drop, must have polarity reversal to turn them off, require a onehalf cycle turn-off time, and have high holding currents and high reverse leakage currents. Thus, thyristor devices are generally unsatisfactory for applications such as general purpose instrument switching which continues to rely on reed switches. The use of anti-parallel connected thyristors is also disclosed in U.S. Pat. No. 4,296,331.
Solid state relays employing a MOSFET rather than thyristor forms an excellent solid state analog of the ideal conduction/blocking characteristics of a pair of mechanical contacts. Bidirectional conduction MOSFETs can control either a-c or d-c circuits, thereby forming a truly universal contact.
One typical prior art solid state relay is that shown in U.S. Pat. No. 4,227,098 which employs a conventional bidirectional conduction MOSFET which is driven into conduction from the output of an optocoupler which provides dielectric insulation between the input and output circuits of the relay. This device, however, has the conventional voltage limitation of bidirectional MOSFETs and can only be used as a low voltage device (for example, lower than about 20 volts) since the device voltage is limited by the breakdown voltage of the gate oxide. The above patent also requires external schottky devices which will break down before the PN junction between the substrate and main electrodes becomes forward-biased to cause a parasitic bipolar transistor to begin conducting. The device of the above patent is also a low speed turn-off device since the very high input impedance required for high speed turn-on also limits the turn-off speed of the circuit.
In order to overcome the inherent low voltage limitation on bidirectional field effect transistors, it is common to employ a vertical conduction power MOSFET in which the source and gate are always close to one another in voltage. Such vertical conduction high voltage MOSFETs, however, are unidirectional conducting devices and thus are useful only for d-c applications, unless at least two such devices are connected to permit bidirectional conduction. A high-voltage solid state relay having integrated components and driven by the output of a photovoltaic device is disclosed in U.S Pat. No. 4,390,790. The circuit shown in this patent employs two vertical unidirectional power MOSFETs connected in circuit arrangement to define a bidirectional current path. Two separate gate drives are required and, moreover, two separate photovoltaic sources are required, one for turning on the two MOSFETs and the other for driving a depletion JFET to produce a discharge circuit for the main power MOSFET gate circuits in an acceptably short time.
Other relay circuits of this general type are shown in U.S. Pat. No. 4,303,831 and 4,307,298.